Although water-borne cationic resin systems are well know, the use of latex binders in cathodic electrocoating is quite new and has not yet achieved full commercial acceptance. Coassigned U.S. Ser. No. 513,621 filed July 14, 1983, now U.S. Pat. No. 4,512,860, which gives background information on electrocoating and especially latex for cathodic deposition, is incorporated herein by reference. Also incorporated by reference are coassigned U.S. Ser. No. 716,665 filed Mar. 27, 1985; and U.S. Ser. No. 716,664 filed Mar. 27, 1985, now U.S. Pat. No. 4,579,889, which relate to the stabilization of cationic latices and paints suitable for electrocoating. The term "latex" is defined as a polymer or copolymer prepared from one or more monomers in an aqueous environment by emulsion polymerization techniques. Such latex, usually having an average particle size from about 400 .ANG. to about 10,000 .ANG. and an average molecular weight ranging from about 10,000 up to and above 250,000, is quite different from the water-reducible or ionizable polymers prepared, not in aqueous medium, but in solvent systems or neat. The latter water-reducible, cation-active polymers have been in commercial use for electrodeposition coatings for some time. The latex polymers and cation-active latices of the present invention are preferred over the commercial water-borne or water-reducible coatings. Such cationic latices are known to be inherently less stable than their anionic counterpart systems. For electrodeposition coatings, it is necessary to develop a latex that can be pumped and will be relatively insensitive to shear forces.
Stability to shear is a necessary property for an electrocoating bath. In a commercial operation, the coating is continuously sheared by centrifugal pumping which passes the material through ultrafiltration membranes at a rate of 35-40 gallons/min. Instability of the coating, leading to agglomeration of particles after shear, would cause fouling of the ultrafiltration membrane, application problems, and loss of coating properties. The term "stable cationic latex" or "stable cationic latex paint composition" in the context of the present invention means a latex that when subjected to shear forces and/or pumping will be substantially non-agglomerated as determined by the HB/DCP shear test using a disc centrifuge described hereinafter.
U.S. Pat. No. 3,640,935 (Abriss) teaches a method of improving latex stability by adding to the latex a nonionic surface-active agent (0.5-3%) as a solution in a water-soluble glycol. Heretofore a prepared latex was judged to be stable if it did not set to a gel after thirty minutes in the Hamilton Beach shear test. It now appears this test is not critical enough to determine particle agglomeration and gelling tendency. A HB/DCB shear test using a disc centrifuge (cf U.S. Pat. No. 4,311,039) was developed to critically examine latex products and to more critically access the degree of latex agglomeration. It was found that cationic latices often readily agglomerate when subjected to shear. Attempts to improve mechanical stability by increasing the cationic precursor amine monomer content or by the addition of conventional surfactants or additives, were insufficient to upgrade the latex to prevent severe agglomeration as determined by the new HB/DCB shear test method. Higher amine monomer level in the latex synthesis quite often diminishes latex conversion and increases dirt (coagulum) levels. In the instant invention improved latex stabilization and cleanliness have been achieved by an improved process for incorporating the crosslinker in an emulsion polymerization using a mixed aqueous/organic medium.